Problem: Let $X$ and $Y$ be sets: $X = \{1,9,6\}$ $Y = \{3,11,1,9,12\}$ What is the set $X \cap Y$ ?
Solution: Remember that $\cap$ refers to the intersection of sets. The intersection of two sets $X$ and $Y$ is the collection of elements which are in $X$ and also in $Y$ The members of a set must be unique, and the order doesn't matter. $X \cap Y = \{1,9\}$